Pathophysiology
Clinical meaning
The motor and sensory examination systematically evaluates the integrity of anatomically distinct neural pathways from the cerebral cortex to the peripheral effectors. Motor pathways involve a two-neuron chain: the upper motor neuron (UMN) originates in the primary motor cortex (precentral gyrus), descends through the posterior limb of the internal capsule, decussates at the medullary pyramids (90% cross contralaterally), and travels in the lateral corticospinal tract to synapse on anterior horn cells. The lower motor neuron (LMN) extends from the anterior horn cell through the ventral root, spinal nerve, and peripheral nerve to innervate skeletal muscle at the neuromuscular junction. UMN lesions produce spastic paralysis, hyperreflexia, positive Babinski sign (upgoing plantar response), and clonus because descending inhibitory modulation is lost. LMN lesions produce flaccid paralysis, hyporeflexia, fasciculations, and muscle atrophy because the final common pathway to the muscle is interrupted. Sensory pathways ascend through two major tracts: the dorsal column-medial lemniscus (DCML) pathway carries proprioception, vibration, and fine touch ipsilaterally up the spinal cord, crossing at the medulla to reach the thalamus and somatosensory cortex; the spinothalamic tract carries pain and...